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The impact of the asteroid (KPg impact event) at Chicxulub is now a well-documented geologic event which took place at the Cretaceous/Paleogene (KPg) boundary (Schulte et al., 2010). However, the effect of this event is relatively unknown in the Tampico-Misantla Basin, which is only about 900 km to the west of the impact site.

In the detailed well reports in the Tampico-Misantla Basin, it is noted that a “brecha” (breccia) is often described at the top of the Cretaceous in many of the wells.. The breccia is described as being gray or white, containing mudstone clasts, having a sandy matrix, recrystallized Globotruncana, and traces of chert, amber, and bentonite (for example, in the Marques-1 well). Early geologists thought the breccia had been deposited in response to the Laramide uplift of the Sierra Madre Oriental. To our knowledge, none of the 100 project wells cored the breccia.

The same KPg breccia crops out in the southern part of the Tampico-Misantla Basin to the southwest of the town of Martinez de la Torre (Figs. 1 and 2). Here, the breccia is a clast-supported conglomerate consisting of cobbles and boulders of limestone, sandstone (medium to coarse grained), and quartz. The matrix is a medium- to coarse-grained sandstone. The KPg contact has been documented just to the west of this outcrop (Mark Bitter, personal communication). The limestone clasts are thought to have been derived from the Tuxpan platform to the northeast, and the sandstone clasts are thought to have been derived from the Sierra Madre Oriental to the west by the backwash of the tsunami generated by the impact event.

In many of the well reports, the wellsite geologists also note that the “Velasco Formation” overlies this breccia. The Velasco Formation is always described as a shale, red, gray, or brown, and compacted. The Velasco Formation has been cored in the Entabladero-101 well from 1140-1149 m and the core has been described as compacted grey/brown shale (Fig. 3). It is devoid of sand.

In this study, the presence and thickness of both the breccia and the Velasco Formation were noted and mapped from the well reports. The wells were drilled between 1936 and 2010 and the early wellsite geologists were probably not always aware of the detailed stratigraphic sequence and certainly not aware of the relevance of the breccia in the basin. The thicknesses of both the breccias and Velasco Formation were estimated from the cuttings descriptions in the wells.

The breccia is absent in the northern third of the Chicontepec Basin. The thickness of the breccia deposit varies between 4 m and 38 m, and is generally about 10-15 m thick. The deposit is fan-shaped, the source is interpreted to be from the northeast (Tuxpan platform), and it pinches out to the southwest (Fig. 4). The only carbonate source area that is present to the northeast is the Faja de Oro atoll, an Albian age reef complex. Additionally, the distribution of the Velasco Formation seems to mimic the distribution of the breccia, albeit covering a slightly larger area. The Velasco Formation varies in thickness between 16 m and 145 m, but it is generally in the range of 30 to 40 m thick.

It is proposed that the breccia plus the Velasco Formation are actually a “megabed” created by the huge tsunami (estimated by some authors to have been over 300 m high) from the KPg impact event (Fig. 5). Many other megabeds around the world show these same characteristics (Cossey and Ehrlich, 1979). The breccia would represent the basal Bouma “A,” or graded division. The Velasco Formation would represent the muddy top, or Bouma “E” division. A good analog for this megabed is from the Jurassic of northern Tunisia (Cossey and Ehrlich, 1979) where a carbonate megabed up to 90 m thick is exposed.

Prior to the KPg impact event, the Tampico-Misantla Basin is primarily a site of carbonate deposition throughout the Cretaceous. Afterwards, the Chicontepec Basin forms as a foredeep in front of the rising Sierra Madre Oriental to the west. The overlying Paleocene Chicontepec Formation consists of turbidite sands composed of over 50% carbonate material (Bitter, 1993) derived from the uplift and erosion of the Sierra Madre Oriental.

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